scsi: target: iblock: split T10 PI SGL across command bios

When T10 PI is enabled on a backing device for the iblock backstore, the PI
SGL for the entire command is attached to the first bio only.  This works fine
if the command is covered by a single bio, but can result in ref tag errors in
the client for the other bios in a multi-bio command, e.g.

[   47.631236] sda: ref tag error at location 2048 (rcvd 0)
[   47.637658] sda: ref tag error at location 4096 (rcvd 0)
[   47.644228] sda: ref tag error at location 6144 (rcvd 0)

The command will be split into multiple bios if the number of data SG elements
exceeds BIO_MAX_PAGES (see iblock_get_bio()).

The bios may later be split again in the block layer on the host after
iblock_submit_bios(), depending on the queue limits of the backing device.
The block and SCSI layers will pass through the whole PI SGL down to the LLDD
however that first bio is split up, but the LLDD may only use the portion that
corresponds to the data length (depends on the LLDD, tested with scsi_debug).

Split the PI SGL across the bios in the command, so each bio's
bio_integrity_payload contains the protection information for the data in the
bio.  Use an sg_mapping_iter to keep track of where we are in PI SGL, so we
know where to start with the next bio.

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>